The invention relates to a support means for the tread of a tire. Said support means constitutes with said tire and its mounting rim a travelling assembly for a vehicle, intended to be able to travel after a consequent and unexpected loss of pressure of the tire, said tire being more particularly a tubeless tire of the heavy-vehicle or civil-engineering type.
French Application FR 2 756 221 describes and claims as support means for a tread a toric membrane of reinforced rubber, inflated to a pressure p0 greater than the pressure p1 of the cavity of the tire, and having, in the inflated state, a crown radius RM less than the loaded radius RE of the tire used at its recommended pressure, said membrane, at least in its crown, being reinforced by at least one ply of cords or cables, said crown of said membrane furthermore comprising at least one hooping reinforcement of circumferentially oriented cords or cables having a breaking load per cm of ply at least equal to the product of the crown radius RM and the pressure, per cm2 of surface of said ply, resulting in a tension per cm of ply equivalent to the tension due to the maximum centrifugal force to which the tire can be subjected and permitting the breaking of the hooping cords or cables for a pressure differential p0xe2x88x92pxe2x80x21, existing in the event of a loss of pressure experienced by the tire, greater than the initial pressure differential p0xe2x88x92p1, that is to say, during normal travel. Said hooping reinforcement may be composed of at least one ply, generally located among the crown reinforcement plies, or of several bands located in the recesses formed on the protective layer or support band radially covering the subjacent plies.
The internal pressure p0 of said membrane, measured in the cold state, that is to say at 20xc2x0 C., is greater than the pressure p1 of the inner cavity of the tire by an amount between 0.5xc3x97105 Pa and 5.0xc3x97105 Pa, depending on the tire dimensions concerned. Given that the crown radius RM of the toric membrane is preferably between 0.80 and 0.97 times the loaded radius RE of the tire, mainly for reasons of heating of said tire, too high a pressure difference risks adversely affecting a certain number of properties of the tire itself, for example the life of the carcass reinforcement of said tire, while requiring an excessively large hooping reinforcement.
The crown of said membrane is preferably reinforced by two plies of cords or cables which are parallel to each other in each ply and crossed from one ply to the next, forming an angle of between 50xc2x0 and 85xc2x0 with the circumferential direction. The cables or cords are advantageously textile ones for reasons of lightness, flexibility and good corrosion resistance, and preferably are made of aromatic polyamide. The axial ends of the two plies are preferably located on the sidewalls of the membrane, such that, if S is the maximum axial width of the carcass reinforcement of the tire, the width of the plies is preferably between S and 1.30 S.
The pressure difference p0xe2x88x92p1 increases in the event of the tire puncturing and the cables of the hooping ply or plies breaking, and the toric supporting membrane expands into the cavity of the tire and enables the whole to continue travelling despite the drop in pressure in the cavity of the tire.
The toric membrane according to the invention described in, the above French application may possibly comprise sidewalls each reinforced by at least one ply of radial cords or cables, said sidewalls possibly being provided advantageously with radial grooves opening on to the metal mounting rim of the tire.
Under normal conditions of travel of the assembly formed by the tire, its mounting rim and the membrane, loading conditions, pressure conditions and speed conditions recommended for the tire in question, the membrane retains an equatorial radius which is practically constant and less than the loaded radius of the tire, and the outer walls of its sidewalls are for a very major part in-permanent contact with the inner walls of the tire. The friction existing between said walls results in premature degradation and wear of the impermeable layer of rubber which covers the inner wall of the tire.
French Application 97/16450, in order to overcome the disadvantages referred to above, proposes imparting to the membrane a particular sidewall architecture in that the sidewall reinforcement ply is wound on either side around an annular reinforcement element, the makeup of which permits breaking after the breaking of the hooping plies, said sidewall reinforcement ply having a meridian profile in the inflated state which is adapted such that there is no contact between the membrane and the inner wall of the closest sidewall from a certain height onwards. In other words, the presence of an annular reinforcement element makes it possible, in the inflated state and during normal travel, to maintain the desired meridian profile of the sidewall reinforcement ply or plies, while not preventing normal, complete expansion of the supporting membrane upon loss of pressure in the tire cavity.
The presence of an annular element or winding bead wire of the sidewall reinforcement plies of the membrane results in two major disadvantages:
it is difficult, not to say impossible, to control the position of the pieces of bead wire after breaking: in particular, accentuated asymmetry, relative to the equatorial plane, of the positioning of the crown of the membrane after expansion, has thus been very frequently observed, which causes certain driving problems for the vehicle thus equipped;
the breaking of the winding bead wires, in the case of a so-called closed supporting membrane (the cross-section through said membrane has a continuous contour), causes great extension of the membrane portions located radially beneath said bead wires, which results in breaking of said portions and escape of inflation gas from said membrane.
In order to overcome the disadvantages referred to above, while retaining, firstly, the principle of an expandable toric supporting membrane capable of filling the cavity of the tire when the inflation pressure of said cavity decreases and/or is cancelled out, and secondly the principle of the lowest possible length of contact between the membrane and the inner wall of the tire in the case of normal travel, the reinforced rubber membrane according to the invention
used as a tread support means for a tire P, and forming with said fire P and its mounting rim J, the nominal diameter of which is DS and the flange of which has an external diameter DR, a travelling assembly which is capable of travel when the tire is subject to a loss of pressure,
inflated to a pressure p0 greater than the pressure p1 of the cavity of the tire,
and having, in the inflated state, a crown radius RM less than the loaded radius RE of the fire used at its recommended pressure, said membrane being
in its crown, reinforced by at least two plies of cords or cables parallel to each other in each ply and crossed from one ply to the next, at least one of said plies being a so-called hooping ply composed of at least one layer of cords or cables, oriented circumferentially and having a breaking load per cm of ply making it possible firstly to resist the tension due to the maximum centrifugal force to which the tire is subjected, increased by the tension due to the pressure differential p0xe2x88x92p1 existing during normal travel, and secondly the breaking of the cords or cables for a pressure differential p0xe2x88x92pxe2x80x21 greater than p0xe2x88x92p1, and
reinforced in each of its sidewalls by at least one ply,
is characterized in that, viewed in meridian section,
the sidewall reinforcement ply is anchored in each lower part of the sidewall by winding around an inextensible annular reinforcement element, which does not break under the action of the tension induced by the action of the sidewall ply and the internal diameter D of which is between the value DR and a value equal to DS minus the product of the width LS of the rim seat and the tangent of the angle of inclination of said seat,
said sidewall reinforcement ply being formed of reinforcement elements, each element comprising a core surrounded by a helically wound cord or cable, the said core having a breaking load which is firstly greater than the tension, per element of ply, due to the pressure differential p0xe2x88x92P1 and to the maximum centrifugal force to which said element is subjected, and secondly such that it permits the breaking of said elements of the sidewall ply for a pressure differential p0xe2x88x92pxe2x80x21, greater than p0xe2x88x92P1 and after the breaking of the elements of the hooping ply,
and said reinforcement element of the sidewall ply having a curve, representing the tension as a function of the relative elongation xcex5, having, for an elongation greater than the elongation at break xcex5A of the core, a segment in which the variation in the tractive force as a function of the elongation is substantially zero,
the elongation at break xcex5R of the reinforcement element being at least equal to the ratio of the difference between the internal meridian length of the tire and the external meridian length of the membrane to the external meridian length of the membrane.
Preferably, the internal diameter D of the inextensible annular reinforcement element lies between the value DS and the value equal to DS minus the product of the width LS of the rim seat and the tangent of the angle of inclination of said seat.
The membrane crown is said to be reinforced by a ply when the presence of said-ply at the crown is effective, whatever the real width of the ply. A membrane sidewall is said to be reinforced by a ply if there is actually a ply in the sidewall; thus a ply anchored to the two membrane bead wires is a sidewall reinforcement ply but also a crown reinforcement ply.